2. Pre-mRNAs, containing
the sequences coding for the ribosomal proteins, are
transcribed by RNA polymerase II
in the nucleoplasm. The pre-mRNAs are processed (see
splicing) and transported (see transport) as
mRNP-particles to the cytoplasm where
they are picked up by the ribosomesinitiation and
translated elongation
into ribosomal proteins. The mature ribosomal
proteins are then transported (see transport) to the
nucleolus.

3. The ribosomal RNAs
(except 5S rRNA) are transcribed in the nucleolus as
one giant precursor RNA by RNA polymerase I.
The precursor is processed (see rRNA processing) to
18S,5.8S and 28S rRNA. 5S rRNA is transcribed in the
nucleoplasm by RNA polymerase III
and transported to the nucleolus. The rRNAs are
folded and associate with ribosomal proteins to form
the 40S and 60S ribosomal subunits. The subunits are
then transported from the nucleolus to the
cytoplasm.

An example of the Rate of Ribosome
Synthesis:

*HeLa cells (a type of human
tumour cells) divide each 24 hours.
* Each cell contains around 10 million ribosomes,
i.e. 7000 ribosomes are produced in the nucleolus
each minute.
*Each ribosome contains around 80 proteins, i.e. more
than 0.5 million ribosomal proteins are synthesised
in the cytoplasm per
minute.
*The nuclear membrane contains
approximately 5000 pores. Thus, more than 100
ribosomal proteins are imported from the cytoplasm to the
nucleus per pore
and minute. At the same time 3 ribosomal subunits are
exported from the nucleus to the
cytoplasm per pore
and minute.